GROUP IV. PHANEROGAM1A. 



451 



the four so formed divides again, so that eight nuclei are formed, 

 four at the micropylar, and four at the chalazal pole of the 

 macrospore ; one nucleus is then conveyed from each pole to- 

 ward the centre of the macrospore, where the two nuclei meet 

 and fuse into one which is termed the definitive nucleus of the 

 macrospore or embryo-sac. Three nuclei now lie at each pole, 

 and around these aggregation of protoplasm takes place, so that 

 cells are formed : those at the chalazal pole soon acquire a cell- 

 wall, and are termed ant ipodal 

 cells: those at the micropylar 

 end do not form any cell- wall ; 

 one of them is the female re- 

 productive cell or oosphere, 

 the other two are sterile 

 (though in rare cases they 

 are fertile), and are termed 

 the synergidce., the three to- 

 gether constituting the egg- 

 apparatus. This is the extent to 

 which the development of the 

 female prothallium takes place 

 previously to fertilisation (Fig. 

 '292). In most Angiosperms 

 the structure of the prothal- 

 lium is completed by the for- 

 mation, after fertilisation has 

 taken place, of additional cel- 

 lular tissue : this process is 

 initiated by the division of 

 the definitive nucleus of the 

 macrospore, nuclear division is 

 repeated, cell-formation takes 



FIG. 292. The female prothallium of An- 

 giosperms, shown in a longitudinal section of 

 the ovule (x 70): ai outer, ii inner, integu- 

 ment ; i micropyle ; / funicle. K Macros- 

 porangium (nucellus). E Macrospore (em- 

 bryo-sac), fc Definitive nucleus of the em- 

 bryo-sac. The female prothallium consists 

 of the egg-apparatus at the micropylar end 

 of the macrospore, and of the group of anti- 

 podal cells at at the chalazal end. The egg- 

 apparatus consists of two synergidae s, and 

 an oosphere e. 



place, in the manner described 



above for the Gymnosperms, 



and the macrospore becomes more or less completely filled with 



cellular tissue, commonly termed endosperm. 



In a number of dicotyledonous plants (e.g. Loranthacese. Orobanchaceae, 

 Labiates, Campanulaceffi) where the embryo-sac is long and narrow, the endo- 

 sperm is developed by cell-division : the embryo-sac is divided by two or more 

 transverse septa and longitudinal divisions follow. And even when the de- 

 velopment of the endosperm begins with free cell-formation, its further develop- 

 ment is effected by the growth and division of tbe first-formed cells. 



